Ab initio investigation of the structure and spectroscopy of hydronium-water clusters

Ab initio (MP2, CASSCF, CASPT2) and DFT/B3LYP calculations have been performed to explore the structures and the electronic and vibrational spectra of hydronium-water clusters as well as the corresponding cluster cations. Minimum-energy and transition-state structures have been optimized at the MP2 and DFT/B3LYP levels. Whereas protonated water clusters can exist both in Eigen-type structures, H3O+(H2O)(n), as well as in Zundel-type structures, H5O2+(H2O)(n), the neutral radical clusters are found to prefer Eigen-type structures, H3O(H2O)(n). While H3O(H2O)(3), like the hydronium radical, is a metastable species, it exhibits a significantly higher barrier for hydrogen detachment (7 kcal/mol), indicating the possibility of kinetic stability of larger H3O(H2O)(n) clusters at low temperatures. Remarkably, hydronium-water clusters are charge-separated species, consisting of a hydronium cation and a localized electron cloud. which are connected by a water network. The vertical electronic excitation energies of the various H3O(H2O)(n) clusters are found to scatter between 1.1 and 3.0 eV, thus covering the energy range of the absorption spectrum of the hydrated electron. The vibrational spectra of H3O(H2O)(n) clusters exhibit the known fingerprint lines of H3O+ and of free OH groups interacting with an excess electron and thus provide direct evidence of their charge-separated character. The relevance of these findings for the radiation chemistry of water is briefly discussed.